Characterising marine heatwaves in the Svalbard Archipelago and surrounding seas

In the Arctic Ocean, satellite-based sea surface temperature data shows that marine heatwave (MHW) intensity, frequency, duration and coverage have increased significantly in recent decades, raising concern for Arctic ecosystems. A high frequency (more than three events per year) of MHWs has been shown around the Svalbard Archipelago. Based on this, we investigate MHW trends around Svalbard at the surface and subsurface, using a regional reanalysis from TOPAZ (1991–2022). We find an increase in the frequency and duration of MHW events around the Svalbard Archipelago over the last decade. Focussing on a region west of Svalbard, we observe an increase in MHW frequency and duration, associated with a long-term rise in sea surface temperature in the region. Analysis of eight individual summer (June–September) MHW events lasting longer than 10 d west of Svalbard, indicated the presence of four shallow (≤50 m) and four deep (>50 m) MHWs after 2010, with a mean duration of 29 d. Some events extended into the Barents Sea. Heat budget analysis demonstrated a greater contribution of ocean heat transport compared to air-sea heat fluxes in driving the MHW events. Deep and shallow events were associated with ocean heat transport anomalies of up to 9 TW. This new understanding of MHW characteristics, including their horizontal and vertical distribution, is key to assessing ecological impacts.